This invention relates to imaging media. In a preferred form, it relates to supports for photographic, inkjet, thermal, and electrophotographic media.
In order for a print imaging support to be widely accepted by the consumer for imaging applications, it has to meet requirements for preferred basis weight, caliper, stiffness, smoothness, gloss, whiteness, and opacity. Supports with properties outside the typical range for imaging media suffer low consumer acceptance.
In addition to these fundamental requirements, imaging supports are also subject to other specific requirements depending upon the mode of image formation onto the support. For example, in the formation of photographic paper, it is important that the photographic paper be resistant to penetration by liquid processing chemicals, failing which, there is a stain present on the print border accompanied by a severe loss in image quality. In the formation of photo quality inkjet paper, it is important that the paper is readily wetted by ink and that it exhibits the ability to absorb high concentrations of ink and dry quickly. If the ink is not absorbed quickly, the elements block or stick together when stacked against subsequent prints and exhibit smudging and uneven print density. For thermal media, it is important that the support contain an insulative layer in order to maximize the transfer of dye from the donor which results in a higher color saturation.
It is important, therefore, for an imaging media to simultaneously satisfy several requirements. One commonly used technique in the art for simultaneously satisfying multiple requirements is through the use of composite structures comprising multiple layers wherein each of the layers, either individually or synergistically, serves distinct functions. For example, it is known that a conventional photographic paper comprises a cellulose paper base that has applied thereto a layer of polyolefin resin, typically polyethylene, on each side, which serves to provide waterproofing to the paper and also provides a smooth surface on which the photosensitive layers are formed. In another imaging material, as in U.S. Pat. No. 5,866,282, biaxially oriented polyolefin sheets are extrusion laminated to cellulose paper to create a support for silver halide imaging layers. The biaxially oriented sheets described therein have a microvoided layer in combination with coextruded layers that contain white pigments such as TiO2 above and below the microvoided layer. The composite imaging support structure described has been found to be more durable, sharper, and brighter than prior art photographic paper imaging supports that use cast melt extruded polyethylene layers coated on cellulose paper. In U.S. Pat. No. 5,851,651, porous coatings comprising inorganic pigments and anionic, organic binders are blade coated to cellulose paper to create photo quality inkjet paper.
In all of the above imaging supports, multiple operations are required to manufacture and assemble all of the individual layers. For example, photographic paper typically requires a paper-making operation followed by a polyethylene extrusion coating operation, or, as disclosed in U.S. Pat. No. 5,866,282, a paper making operation is followed by a lamination operation for which the laminates are made in yet another extrusion casting operation. There is a need for imaging supports that can be manufactured in a single in-line manufacturing process while still meeting the stringent features and quality requirements of imaging bases.
It is also well known in the art that traditional imaging bases consist of raw paper base. For example, in typical photographic paper as currently made, approximately 75% of the weight of the photographic paper comprises the raw paper base. Although raw paper base is typically a high modulus, low cost material, there exist significant environmental issues with the paper manufacturing process. There is a need for alternate raw materials and manufacturing processes that are more environmentally friendly. Additionally to minimize environmental impact, it is important to reduce the raw paper base content, where possible, without sacrificing the imaging base features that are valued by the customer, i.e., strength, stiffness, surface properties, and the like, of the imaging support.
An important corollary of the above is the ability to recycle photographic paper. Current photographic papers cannot be recycled because they are composites of polyethylene and raw paper base and, as such, cannot be recycled using polymer recovery processes or paper recovery processes. A photographic paper that comprises significantly higher contents of polymer lends itself to recycling using polymer recovery processes.
Existing composite color paper structures are typically subject to curl through the manufacturing, finishing, and processing operations. This curl is primarily due to internal stresses that are built into the various layers of the composite structure during manufacturing and drying operations, as well as during storage operations (core-set curl). Additionally, since the different layers of the composite structure exhibit different susceptibility to humidity, the curl of the imaging base changes as a function of the humidity of its immediate environment. There is a need for an imaging support that minimizes curl sensitivity as a function of humidity, or ideally, does not exhibit curl sensitivity.
The stringent and varied requirements of imaging media, therefore, demand a constant evolution of material and processing technology. One such technology known in the art as polymer foams has previously found significant application in food and drink containers, packaging, furniture, appliances, and the like. Polymer foams have also been referred to as cellular polymers, foamed plastic, or expanded plastic. Polymer foams are multiple phase systems comprising a solid polymer matrix that is continuous and a gas phase. For example, U.S. Pat. No. 4,832,775 discloses a composite foam/film structure which comprises a polystyrene foam substrate, oriented polypropylene film applied to at least one major surface of the polystyrene foam substrate, and an acrylic adhesive component securing the polypropylene film to said major surface of the polystyrene foam substrate. The foregoing composite foam/film structure can be shaped by conventional processes as thermoforming to provide numerous types of useful articles including cups, bowls, and plates, as well as cartons and containers that exhibit excellent levels of puncture, flex-crack, grease and abrasion resistance, moisture barrier properties, and resiliency.
Foams have also found limited application in imaging media. For example, JP 2839905 B2 discloses a 3-layer structure comprising a foamed polyolefin layer on the image-receiving side, raw paper base, and a polyethylene resin coat on the backside. The foamed resin layer was created by extruding a mixture of 20 weight % titanium dioxide master batch in low density polyethylene, 78 weight % polypropylene, and 2 weight % of Daiblow PE-M20 (AL)NK blowing agent through a T-die. This foamed sheet was then laminated to the paper base using a hot melt adhesive. The disclosure JP 09127648 A highlights a variation of the JP 2839905 B2 structure, in which the resin on the backside of the paper base is foamed, while the image receiving side resin layer is unfoamed. Another variation is a 4-layer structure highlighted in JP 09106038 A. In this, the image receiving resin layer comprises 2 layers, an unfoamed resin layer which is in contact with the emulsion, and a foamed resin layer which is adhered to the paper base. There are several problems with this, however. Structures described in the foregoing patents need to use foamed layers as thin as 10 xcexcm to 45 xcexcm, since the foamed resin layers are being used to replace existing resin coated layers to the paper base. The thickness restriction is further needed to maintain the structural integrity of the photographic paper base since the raw paper base is providing the stiffness. It is known by those versed in the art of foaming that it is very difficult to make thin uniform foamed films with substantial reduction in density especially in the thickness range noted above.
Another key feature of imaging media is bending stiffness. It is well known that stiffness of an imaging element is a function of the modulus of the various layers of the imaging element, the location of the various layers (particularly in terms of the distance from the bending axis) and the overall caliper of the imaging element. Improvements that can be made to the modulus of the various layers comprising the imaging element can increase the overall bending stiffness of the element thus, in turn, increasing its value as an imaging support.
Organic additives that have the potential to enhance the modulus of a polyolefin film are known in the art. The composition of the organic additive, which is typically a hydrocarbon resin, must be such that it exhibits a higher glass transition temperature (Tg) than polyolefin, for example, propylene. It must also be compatible with polyolefins such as propylene. It is believed that the addition of the organic additive increases the Tg of the amorphous polyolefin, leading to a densification of the amorphous phase over time, which leads to increased stress transfer between crystalline regions (also called a pseudonetwork effect) that, in turn, leads to increasing stiffness. For example, Bossaert et al. in U.S. Pat. No. 4,921,749 claim a polyolefin film comprising a base layer of 70% to 97% polypropylene and 30% to 3% hydrogenated resin. The addition of about 20% hydrogenated resin is shown to result in an increase in modulus of about 10-20%. Klosiewicz, in U.S. Pat. No. 6,281,290 claims a process for producing a master batch for a polypropylene article (film, fiber, sheet, or bottle) comprising a mixture of polypropylene, high density polyethylene and hydrocarbon resin having a Ring and Ball softening point of at least 70 degrees Centigrade. The addition of low levels of hydrocarbon resin and high density polyethylene (HDPE) are reported to increase the tensile modulus of extrusion cast polypropylene films by 15% to 70%. C-S Liu, in U.S. Pat. No. 4,365,044, discloses an extrusion-coatable polypropylene composition comprising a hydrogenated copolymer of vinyl toluene, alpha-methyl styrene, and low density polyethylene. Extrusion coatability at speeds up to about 900 feet per minute (274 meters/min.) and good adhesion to cellulose substrates is claimed.
There is a need for a composite material that can be manufactured in a single in-line operation and that meets all the requirements of an imaging base, especially the bending stiffness requirements. There is also a need for an imaging base that reduces the amount of raw paper base that is used and can be effectively recycled. There is also a need for an imaging base that resists the tendency to curl as a function of ambient humidity.
These and other objects of the invention are accomplished by an imaging member comprising an imaging layer and at least one stiffening layer comprising a blend of polyolefin polymer and amorphous hydrocarbon resin having a softening temperature of greater than 30 degrees Centigrade. The invention further describes a method for making the imaging member, comprising extruding a foam polymer sheet, orienting the foam polymer sheet, bringing a stiffening layer comprising a blend of polyolefin polymer and amorphous hydrocarbon resin into contact with the oriented foam polymer sheet, and applying an imaging layer above the stiffening layer. A second method of forming an imaging member comprises extruding a foam polymer sheet, bringing at least one stiffening layer comprising a blend of polyolefin polymer and amorphous hydrocarbon resin into contact with the foam polymer sheet, orienting said foam polymer sheet and said stiffening layer and applying an imaging layer above said stiffening layer. Another method describes the formation of an imaging member comprising making a cellulosic sheet, bringing at least one stiffening layer comprising a blend of polyolefin polymer and amorphous hydrocarbon resin into contact with the cellulosic sheet and applying an imaging layer above the stiffening layer.
This invention provides a superior imaging support. Specifically, it provides an imaging support of high stiffness, excellent smoothness, high opacity, and excellent humidity curl resistance. In one embodiment, it also provides an imaging support that can be manufactured using a single in-line operation that can be effectively recycled.
The present invention offers several advantages. The invention produces an element or member that has much less tendency to curl when exposed to extremes in humidity. In one embodiment, the ability to manufacture the element or member in a single in-line operation significantly lowers element manufacturing costs and may eliminate disadvantages in the manufacturing of the current generation of imaging supports, such as very tight moisture specifications in the raw base and specifications to minimize pits during resin coating. In one embodiment, the element or member can also be recycled to recover and reuse polyolefin instead of being discarded into landfills.